Effects of the combined pollution of Pb2+ and Cd2+ on soil urease activity were shown in Table 2.
Compare with the controls, the enzymes activities were decreased with the increasing concentrations of
Cd2+ and the incubation periods except for treatments of 0.5 mg/kg Cd2+ only and 0.5 mg/kg Pb2+ and 0.5
mg/kg Cd2+ combined. Urease activities were found to be sensitive to the inhibition effect of heavy metals.
After 45 days incubation under the concentrations of 100.0 mg/kg Pb2+ and 0.5 mg/kg Cd2+ combined, the
inhibition rates of soil urease activity was determined at 73.1 % compared to the control.
Disagree with the results of Zhou et al. [21], soil enzyme activity was found to be a different degree of
recovery after a long period of heavy metals stress in their research, which would be contributed to the
different concentrations of heavy metals. The inhibition effect of heavy metals to soil enzyme activities
was the results of the changes of chemical conformation mainly due to the coordination reaction. Based
hard and soft acids and base theory (HSAB), the active sites in enzyme protein molecular,
such as thiol or imidazolyl groups, were preferred coordinated with soft heavy metals.
Table 2. Effects of the heavy metals pollution on soil urease activity .